The field of the present invention is surge control for centrifugal compressors.
Centrifugal compressors can be susceptible to the phenomenon of surging. Surging is typically found to occur at specific ranges of flow for each compressor system. The range of flow may be located experimentally and efforts undertaken to avoid that range. The surge point is also affected by the speed of the compressor. To avoid such surge, bypass systems have been used which include a flow path around the compressor which can, for example, return compressed air to the compressor inlet to increase the actual flow through the compressor to a level avoiding the surge point. Bypass valves have been used in such flow paths to control the system.
Two common methods have been employed for sensing the onset of surge and actuating a bypass valve to avoid the phenomenon. In a first system, the surge flow range for a compressor system is experimentally located. Instrumentation may then be employed to generate a signal when the compressor approaches the critical range and to operate a bypass system responsive to the onset of surge. Typically this instrumentation senses the pressure difference generated by the compressor. This pressure difference varies approximately as the square of the compressor speed. Thus, a surge onset line plotted against pressure and flow appears as a parabola. As the use of a parabolic curve is difficult, a conventional approach is to use the pressure drop across a flow meter which varies as the square of flow and, therefore, also as the square of speed. This ratio of the compressor pressure gain and the drop in pressure across the flow meter is, therefore, relatively constant regardless of flow and speed. This ratio thus becomes useful to control a surge preventing bypass valve through comparison with an empirically determined constant.
Another conventional method for controlling surge is by means of instrumentation that can sense pulsations. Characteristic pulsations can be observed which signal the onset of surge. Again, a bypass valve can be controlled to artificially change flow conditions through the compressor to avoid the critical flow range.
Compressor systems where surge becomes a concern typically have a compressor pressure gain ratio approaching two. The compressor head gain varies as the square of compressor speed. This head gain is related to the pressure rise as follows: ##EQU1## The value of ln (P.sub.2 /P.sub.1) for small values of P.sub.2 /P.sub.1 approaches (P.sub.2 P.sub.1)/P.sub.1, illustrated by the following table:
______________________________________ P.sub.2 /P.sub.1 1.001 1.1 1.2 1.5 2.0 3.0 10.0 ln (P.sub.2 /P.sub.1) 0.001 .095 .182 .405 .690 1.099 2.303 ##STR1## .999 .950 .910 .810 .690 .550 .26 ______________________________________
From the foregoing, it can be seen that for infinitesimal pressure rise ratios, the ratio of (P.sub.2 -P.sub.1)/P.sub.1 is equal to the logarithm of P.sub.2 /P.sub.1. In other words, at low compression ratios, the conventional method based on compressor pressure rise is accurate. At the same time, the error rapidly rises with pressure ratio. At a ratio of 1.1:1 the error is 5%; at 2:1 the error is 31%.